A currently popular method of regenerating diesel particulate filters is to use electric resistance heaters. These heaters use the electrical power supply of the engine alternator and batteries to raise the temperature of the filtered particulate matter so that combustion can take place. Large ceramic filters have a relatively high loading capacity and can filter for long periods of time before regeneration is needed. These filters can be regenerated electrically by heating a small portion of the massive filter, and then allowing the regeneration to propagate itself. Larger mass, slow warm-up heater designs can be used because the regeneration duty cycle is very low, and the ratio of heater on-time to loading time is also low. These filters are expensive, and are only cost justified for heavy duty applications.
Diesel filter design concepts for lighter duty applications are moving toward lower cost, lighter weight, modular filter cartridges. These filter cartridges often use a fibrous material that cannot retain enough heat to support self-propagated regenerations. They do not have as much filter media area as ceramic filters, so they require much more frequent regeneration. As such, these filter cartridges require an electric resistance heater that can warm-up very quickly and transfer energy efficiently to the media and the trapped particulate matter. As a consequence, the heater must have exposed elements, must be light weight, must have high surface area for maximum heat transfer, and must be integrated closely with the media and other support structure to enhance heat transfer. Filter cartridge heaters must be partially open to allow unrestricted air flow, and must be able to withstand the extreme thermal stresses associated with close structural contact with other support members. Furthermore, because of the higher regeneration duty cycle, filter cartridge heaters must withstand many thousands of warm-up/cool-down cycles.
Modular filter cartridge design and a multitude of different applications for cartridges require a variety of heater configurations in order to meet power and physical space limitations. Existing heater designs are known by the terms: cal-rod, ribbon, coil, and expanded metal. A cal-rod is essentially an insulative rod containing resistance wire. The rod can be straight or shaped as a spiral or given other shapes. A cal-rod is very heavy, slow to warm-up, and has low surface area with resulting poor heat transfer characteristics.
A ribbon heater is a thin length of resistive material. A ribbon heater has low surface area and cannot be easily integrated with filter media.
A coil heater is a coil of resistive material. A coil heater has low surface area and cannot be integrated with filter media. Thermal growth can cause electrical shorts in coils unless insulated structure is used to keep adjacent coils separated.
An expanded metal heater is shown in FIGS. 1-3. An expanded metal heater has good properties of mass and surface area. It, however, has severe structural deficiencies which lead to high failure rates. In FIG. 1, there is shown a cartridge filter having an expanded metal heater. Briefly, cartridge 100 has a perforated support member 102 and an expanded metal heating element 104 with filter media 106 therebetween. As shown more clearly in FIGS. 2 and 3, the expanded metal heater 104 has sharply notched corners 108. During heat growth and cooling contraction, the sharp corners result in high stress concentrations. Within a relatively few warm-up/cool-down cycles, the high stress often results in breakage of a strand 110. When a first strand breaks, the current paths of adjacent strands receive higher current density causing a higher temperature in the strands which consequently then come under a little more stress. Before long, an adjacent strand breaks, and the process escalates until a few strands have broken. Due to the higher and higher current densities and temperatures, the remainder of the strands eventually fuse open in a melting process. That is, the current density is raised to the point where the temperature of the remaining strands reaches the melting point.
Of further concern in this regard is the region 112 of overlap of one side edge of the expanded mesh over another in order to form the particular shape. Since there are a large number of strands from one end of the expanded wire heating element to the other, it is too costly and difficult to try to match all strands and butt weld them. As a consequence, One side edge is allowed to overlap another and they are spot welded together. The overlapping and spot welding, however, lead to non-uniform current path lengths which can exacerbate current and temperature dissimilarities.
In addition, an expanded metal heater necessarily results in a shape having many parallel resistance paths. Because there are many parallel resistance paths, each path must have proportionately reduced path cross-section in order to meet the total resistance requirement and provide the necessary heat. This results in low structural strength and becomes a factor in the failure problem discussed above.
Also, expanded metal by the very nature of the manufacturing process, results in a diamond shape. The diamond shape reduces current path length from what might otherwise be possible, which is a further reason requiring reduced path cross-section to meet resistance requirements. Further, the diamond shape has a longitudinal stiffness component which does not allow structural flexibility during thermal growth and contraction and is a further factor leading to the failure problem indicated.
The present invention overcomes the limitations of the expanded metal heater and other heaters and results in a heater capable of withstanding the many thousands of warm-up/cool-down cycles required for an acceptable filter cartridge heater.